The present invention relates to receivers and methods for receiving digital radio signals while minimizing distortion, particularly in receivers with hard limiting circuits, and the application of equalizer techniques in wireless telecommunication systems, especially mobile communication systems.
Equalizers are used to eliminate distortion in various receivers. Equalizers are typically used in linear receivers where signal amplitude and phase are preserved.
In the presence of frequency selective fading an equalizer is useful to improve receiver performance. The structure of the equalizer depends on the type of receiver front end. In linear receivers, samples contain channel information. Examining this information can provide an accurate channel estimate for equalization. On the other hand, non-linearity at the front end simplifies the receiver design since channel information is stripped from the amplitude of the received signal. Therefore information can be examined by analyzing with the received signal strength intensity (RSSI) signal or by extracting information from the received signal phase.
There are three types of equalizers: a linear equalizer, a decision feedback equalizer (DFE), and a maximum likelihood sequence estimator equalizer (MLSE). Linear equalizers are not suitable for channels with frequency selective fading, since such fading amounts to significant noise enhancement.
Moreover, decision feedback equalizers do not attempt to compensate for the effect of distortion which might be done by using a cascaded adaptive FIR filter. Instead a nonlinear structure is used to remove the effect of post:cursor intersymbol interference distortion by using previous decisions and a feedback filter. In a decision feedback equalizer, the noise enhancement is less than that of the linear equalizer.